Square-type trimming potentiometer

ABSTRACT

A trimming potentiometer including a housing having top and bottom sections. The bottom section houses a gear which has on one of its flat surfaces a laterally disposed cam for engagement with a matching cam on one surface of a contact drum which also fits within the housing. The drum has on its reverse surface an electrical contact pressed against an arcuate-shaped resistance element housed within the top section and having each of its ends connected to a conductive terminal. The gear is driven by a worm rotatably mounted in the bottom section. Rotation of the worm drives the gear, which rotates the drum, causing the electrical contact to sweep across the arcuate-shaped resistance element. The drum has a stop that engages abutment edges on the housing to limit rotation of the drum so that the electrical contact will not go beyond the arcuate limit of the resistance element, usually 270* or so. If rotation of the screw is continued after the stop engages the abutment edges at either end of the range of rotation, the gear and screw are forced out of engagement with each other by camming action between the cam on the gear and the matching cam on the drum to prevent destruction of either the gear section or the thread surface of the worm. Within the bottom section is positioned a gear spring which will flex when the gear is forced out of position against it, but will keep the gear otherwise engaged with the screw. The drum is generally circular in shape and large enough to have one of its sides ordinarily pressed against a shoulder at the bottom section. Since the drum will rotate with the gear, unless otherwise provided, the drum will encounter large frictional forces when turning against the shoulder of the section. To reduce friction a wave spring is provided between the drum and the section shoulder.

United States Patent [72] Inventor Edward Schoettly Madison, NJ.

[21 1 Appl. No. 73,579

[22] Filed Sept. 18, 1970 [45] Patented Dec. 7, 1971 I [73] Assignee Vishay 1ntertechnology,lnc.

Malvern, Pa.

Continuation-impart of application Ser. No.

7,447, Feb. 2, 1971, now abandoned. This application Sept. 18, 1970, Ser. No. 73,579

[54] SQUARE-TYPE TRIMMING POTENTIOMETER 7 Claims, 5 Drawing Figs.

Primary Examiner- Laramie E. Askin Assistant Examiner-D. A. Tone ABSTRACT: A trimming potentiometer including a housing having top and bottom sections. The bottom section houses a gear which has on one of its flat surfaces a laterally disposed cam for engagement with a matching cam on one surface of a contact drum which also fits within the housing. The drum has on its reverse surface an electrical contact pressed against an arcuate-shaped resistance element housed within the top section and having each of its ends connected to a conductive terminal. The gear is driven by a worm rotatably mounted in the bottom section. Rotation of the worm drives the gear, which rotates the drum, causing the electrical contact to sweep across the arcuate-shaped resistance element. The drum has a stop that engages abutment edges on the housing to limit rotation of the drum so that the electrical contact will not go beyond the arcuate limit of the resistance element, usually 270 or so. If rotation of the screw is continued after the stop engages the abutment edges at either end of the range of rotation, the gear and screw are forced out of engagement with each other by camming action between the cam on the gear and the matching cam on the drum to prevent destruction of either the gear section or the thread surface of the worm. Within the bottom section is positioned a gear spring which will flex when the gear is forced out of position against it, but will keep the gear otherwise engaged with the screw.

The drum is generally circular in shape and large enough to have one of its sides ordinarily pressed against a shoulder at the bottom section. Since the drum will rotate with the gear, unless otherwise provided, the drum will encounter large frictional forces when turning against the shoulder of the section. To reduce friction a wave spring is provided between the drum and the section shoulder.

* PATENTEDUEC nan 35 65 1 sum 1 0F 2' FIG. 2 28 1 more. EDWARD S ETTLY ATTORNEY PATENTED on: 7 l97| SHEEI 2 OF 2 I FIG. 5

- INVENTOR. EDWARD SCHOETTLY ATTORNEY I 1 SQUARE-TYPE TRIMMING POTENTIOMETER CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part application of Ser. No. 7,447 now abandoned, filed Feb. 2, 1970.

Field of the Invention This invention relates to small potentiometers of the type which may be employed for trimming purposes and which are provided with an adjustment. screw to permit precise control through mere rotation of thescrew. The resistance element is laid out in an arcuate locus to permit the potentiometer to be constructed in a compact and squarelike unit.

BACKGROUND OF THE INVENTION In recent years the manufacture of trimining potentiometers has become quite important because of their capacity to provide exact control over resistance in electronic circuits where such control is desirable but where the resistance value need not be changed once a proper value is established. One configuration of such potentiometers is in the form of an elongated enclosure with the resistance element in the form of a flat, straight strip of resistive material. In such potentiometers a lead screw is provided to drive a contact along the length of the resistive element and additional means are provided to establish connection to the contact at every point along its range of travel. However, such elongated potentiometers are not always convenient to use because of the amount of space that they occupy.

' resistive path can be relatively long if it is so formed even though overall maximum dimensions of the potentiometer -may be substantially less than the greatest dimension of a linear potentiometer having a resistive path of comparable length laid out in a straight line. It is still desirable to provide a worm-and-gear drive to achieve the same degree of control over the rotation of a contact arm along the length of the arcuate resistive element as is obtained by the lead screw drive in trimming potentiometers having straight resistive elements.

Because of the rapid growth of components of the type described, several different configurations have been produced. In one configuration there are leads, or rigid support posts, extending from one side of the potentiometer in the axial direction of the lead screw parallel to those leads. In another configuration, the axial dimension of the lead screw may be perpendicular to the support posts.

Potentiometers of the type described are usually made quite small and may have major external dimensions of the order of one-fourth of an inch. Such small components cannot be very strong and means must be provided to keep the unit from breaking if a user attempts to turn the lead screw beyond a position that will bring the contact finger into junction with the arcuate resistive material. Thus, the contact finger may be provided with or attached to, a plate having a projecting stop that intercepts fixed abutment edges at each end of the range of travel of the finger so that the gear can continue to turn as long as a user persists in rotation of the lead screw. The plate and the gear may both be mounted, in accordance with the present invention, on a support post, and resilient means may be mounted, on the same post to force the plate and gear into engagement, but in such a way that the force of the resilient means may be overcome if an attempt is made to rotate the lead screw beyond the point at which the plate engages the stop.

SUMMARY OF THE INVENTION An object of the present invention'is to provide an improved square-type trimming potentiometer.

Another object of the present invention is to provide a simplified trimming potentiometer with an internal subassembly that can be inserted into a cover in which a lead screw may be mounted so as to be perpendicular to or parallel with support.

ate-shaped resistance element removably fixed within the top section member and arranged to communicate with the electrical contact of the drum, means for keeping the contact drum from rotating more than 360 and for keeping the gear engaged with the screw and for flexing with the gear when it is forced out of engagement with the screw member, means for reducing friction when the drum rotates against the bottom section of the housing and electrical means for connecting the highly conductive parts of the resistance element to a power source.

BRIEF DESCRIPTION OF THE DRAWINGS Other and further objects, advantages, and features of the invention will be apparent to those skilled in the art from the following detailed description thereof, taken in conjunction with the accompanying drawings, in which:

FIG. I is a perspective view of a potentiometer constructed according to the invention;

FIG. 2 is a cross-sectional side elevated view of the potentiometer of FIG. 1;

FIG. 3 is a cross-sectional view of the potentiometer of FIG. 2 along the line 33 showing the resistive element;

FIG. 4 is a cross-sectional view of the potentiometer in FIG. 2 along the line 44 showing the drum and cam of FIG. 2; and

FIG. 5 is a cross-sectional view of the potentiometer of FIG. 2 along the line 55 showing the worm and pinion gear in a disengaged position.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The potentiometer, as shown in the drawings, has a bottom section or shell 15 composed of integrally material with a gear 16 (FIGS. 4 and 5) housed in it. Across one end of the bottom section I5 is mounted an adjustment screw 17 housing a lock pin 18 (FIG. 4) at one of its ends and an O-ring 19 (FIGS. 4 and 5) at its other end to keep the adjustment metal screw 17 in position. The triangular-shaped lateral cam 21 (FIGS. 2, 4 and 5) of the gear 16 is positioned within a similarly shaped recessed part of a contact drum 22 (FIGS-2, 4 and 5), which may be made of dialkyl phthalate or other suitable material. The upper side of this contact drum contains a generally rectangular shaped groove into which fits an electrical contact 23 (FIG. 2). This groove may be slightly wider at the bottom to retain the contact 23. The contact drum 22 contains integrally fixed at its periphery a periphical cam 24 and this cam 24 abuts against the cam stop 25 which is integrally a part of the bottom shell I5 (see FIG. 4).

FIG. 2 shows a cross section of the potentiometer. Within the top shell or section 28 is housed a disc-shaped substrate 29 of suitable insulating material, such as vitreous and ceramic material. The substrate 29 is ordinarily easily removable from the top shell 28 when not electrically connected to the terminal leads 30, 31 and 32 (FIG. I) by means of jumpers 34, 35 and 36 (FIG. 3), respectively. As will be explained in greater detail hereinbelow substrate 29-has coated on its surface resistive material to provide the desired potentiometer effect.

Referring particularly to FIG. 3, which is a detailed view of the resistive element 29, it will be noted that in one embodiment of the invention two conductive terminals 37 and 38, each composed of gold or other highly conductive metal, are electrically connected with the terminal leads 30 and 32 (FIG. 1), and are positioned so that inner boundaries form an arc of 54 and the outer boundaries an arc of 909. interconnecting with the terminals 37 and 3B is resistive material coated on the surface of the substrate 29, which material is in one embodiment of the invention 50 percent nickel and 50 percent iron. The substrate itself may be composed of glass or a ceramic material. On the central unit of the substrate 29 is coated a circular-shaped conductive terminal 41 which narrows in the shape of a narrow band 42 for communication with the terminal lead 31 (FIG. 1). The conductive channel of band 42 passes between the tenninals 37 and 38 without touching them. The substrate 29 has a small notch 63 for the purpose of engagement with the butt 43a of the top shell 28 to prevent rotation.

FIG. shows the detail of gear 16. The gear cam 21 rises above the gear and its sides are triangularly shaped forming an angle of 54 with each other. The teeth 45 of gear 16 form an approximate angle of with each other.

FIGS. 2 and 4 show details of the contact drum 22. On one side of the drum 22 is a groove 47 into which is slidably mounted the electrical contact 23. The groove 47 is positioned to provide communication of the contact 23 with the arcuate resistor 40 of substrate 29 and with the central conductive terminal 41. On the reverse side of the drum 22 is a recess 4% which is shaped the same as the lateral cam 21 of gear 16, and will receive the lateral cam 21 when the potentiometer is assembled. Otherwise, drum 22 is generally cylindrical in shape and will fit within the bottom and top shells or sections as previously explained. At one part of the drum 22 is a peripheral cam 26 which serves to prevent the drum 22 from rotating a full 360".

The upper part of the contact 23 is semicylindrical in shape and is in two parts; namely, 50 and 51. One part communicates with the central conductor 41 and the other with the arcuate resistor 40 of the resistance element or substrate 29. The contact 23 may be composed of carbon. The remaining butt 52 of the contact is rectangularly shaped and fits into the groove 47 of the drum 22.

A wave spring 54, which is circular in shape and has free ends 55 and 56, is shown in FIG. 4. The ends form an angle of 35 with each other as shown; The wave spring 54 is not flat and this is due to the three waves 57, 58 and 59. The waves prevent sticking of the spring to the drum 22 and shoulder 60 of the bottom shell 15. The drum 22 will be turned by means of gear 16 and the movement should be preferably smooth because of the wave spring 54.

When the potentiometer is assembled and ready for use, it is simply adjusted to the desired electrical setting by a turn of the screw 17. Once set, the potentiometer may remain fixed or be reset at will. The construction is such that very fine settings are possible with the potentiometer which heretofore have been difficult to attain. The normal operation of the potentiometer is accomplished by turning the adjustment screw 17, which is meshed with the gear 16 to cause the gear to rotate. Due to the engagement between the wedge-shaped cam 21 and the correspondingly wedge-shaped recess 48 in the drum 22, the drum will rotate with the gear. The direction of rotation of the gear and drum depend on the direction of rotation of the adjustment screw 17. When the adjustment screw is turned far enough, which may be several complete revolutions, the peripheral cam 24 strikes one side or the other of the cam stop 25 and prevents the drum 22 from rotating any further. Potentiometers made according to the present invention are so small that the interference between the cam 24 and the stop 25 may not be noticed, particularly since the adjustment screw 17 is normally Operated by a screwdriver which further separates the person adjusting the potentiometer from the effect of having the cam strike the stop.

According to the present invention, further rotation of the adjustment screw is permissible because it will merely cause the gear 16 to rotate, and the pressure of the cam 21 on the appropriate side of the wedge-shaped recess 48 will cause the gear to slide and to press against a flat leaf spring 62. The gear and the leaf spring are both located in a recess 63, as shown in FIG. 5, which is large enough to permit the gear to have some freedom of motion at least in the direction perpendicular to the axis of the adjustment screw 17. The recess 63 also has two shoulders 64 and 65 on which the ends of the flat leaf spring 62 rest, and when the gear 16 moves perpendicularly away from the screw 17 due to interference between the teeth 45 and the thread of the screw, the opposite side of the gear 16 will cause the spring 62 to flex. The flexed position of the screw 62 is shown in FIG. 5, and the relatively unflexed position is shown in FIG. 4, although there is a certain amount of flexure even in that position. This flexure provides the spring force necessary to urge the gear 16 into engagement with the threads of the screw 17.

It will be noted that one surface 66 of the wedge-shaped recess 48 is substantially perpendicular to the axis of the screw 17. This corresponds to the fact that the peripheral cam 24 has been moved clockwise as far as it will go and is in contact with the stop 25. Further rotation of the screw 17 in the same direction will merely cause the engaging teeth 45 of the gear 16 to ride up on the surface of the threads of the screw 17 and slip into the next engaging thread of the screw in response to pressure from the spring 62. It is essential that the surface 66 be substantially perpendicular to the thread of the screw to permit easy movement of the cam 21 along the surface 66.

The same action occurs when the screw 17 is rotated in the opposite direction far enough to rotate the drum as far counterclockwise as it will go, which will bring the peripheral cam 24 into contact with the other surface of the stop 25. The effective angular widths of the stop 25 and of the cam 24 are determined by the angular distance between the terminals 37 and 33 (FIG. 3). The drum 22 must be permitted to rotate far enough to allow the contact 23 to traverse the entire length of the resistive element 40, but not substantially farther than that. When the cam 24 strikes the other surface of the stop 25, the surface 67 (FIG. 5) of the wedge-shaped recess 48 will be perpendicular to the axis of the screw 17 and further rotation of the screw will force the gear 16 out of engagement against the pressure of the spring 62. Thus, when the screw 17 is rotated excessively in either direction the gear will be forced out of engagement but will automatically be returned to engagement as soon as rotation of the screw is reversed. The pressure available from the spring 62 must be sufficient to urge the gear into engagement with the screw 17, but not so great that the teeth of the gear will be subjected to excessive wear if the screw is rotated too far.

Preferably, the contact or contacts which are in engagement with the resistive film are made from precious metal alloys. The exact composition of the precious metal alloy employed is not critical. However, the metal alloy should be selected to provide reasonable wear resistance characteristics and excellent contact. An example of a precious metal alloy which may be used is an alloy having the following composition:

Palladium 35% Silver 30% Copper l3.75% Platinum lO'l: Gold 10% Zinc 1.25%

Another suitable precious metal alloy has the following composition:

Gold 56.25% Copper 2l% Palladium l0.5% Silver 5.25% Nickel 5.25% Zinc I.75%

It will be understood that the number of contacts which are in engagement with the resistive film is not critical.

THe preferredsubstrate for the resistive film of the present invention is a vitreous material, such as glass. Nevertheless, as

indicated herein, ceramic substrates may also be employed. If desired, the resistive metal film can be fired directly on the substrate which is employed. Said film can also be bonded to the substrate. 7

From the foregoing it will be seen that this invention is well adapted to obtain all of the ends and objects hereinabove set forth, together with other advantages which are obvious and which are inherent to the system.-

It will be understood that changes may be be made in the details of construction and arrangement of parts, shown and described herein, without departing from the spirit of the invention as set forth in the accompanying claims, as the preferred forms of the invention have been given by way of illustration only.

What is claimed is:

l. A trimming potentiometer comprising a housing including a top and bottom shell number, a screw member rotatably mounted in the bottom shell member, a gear movably positioned within said bottom shell member and engaging said screw member and having on one of its sides a laterally disposed cam, a contact drum containing on one side an electrical contact and on the others a groove in engagement with the gear cam thereby enabling the gear and drum to rotate as a single unit, a resistance element removably fixed within the top shell member and communicating with the electrical contact of the drum, the resistance element being further characterized by having an arcuate shaped resistor coated on its surface and a highly conductive element coated at each end thereof forming an angle of less than 360 and a central conductor out of contact with the other resistor and conductive parts coated on the center part of the resistance element, means by which the contact drum cannot rotate more than 360 means for keeping the gear engaged with the screw and for flexing with the gear when it is forced out of engagement with the screw member, means for reducing friction when the drum rotates against part of the bottom shell and electrical means for connecting the highly conductive parts of the resistance element to a power source.

2. The apparatus of claim 1 wherein said means for keeping the gear engaged with the screw comprises a spring positioned within the bottom shell in proximity to the gear whereby it will flex whenever the gear is forced out of engagement with the screw member and otherwise serve to keep the gear in engagement with the screw.

3. The apparatus of claim 2 in which said spring is a flat, resilient strip held in contact with the teeth of said gear.

4. The apparatus of claim 1 in which said laterally disposed cam is wedge shaped and is so oriented with respect to said gear, said electrical contact, and said resistor that each flat surface of said cam is substantially perpendicular to said screw member when said gear is in either of two end positions to place said contact upon a respective end of said resistor, whereby further rotation of said screw member will force the cam out of said groove in said contact drum to permit said gear to overcome the pressure of said means for keeping the gear engaged with the screw and move out of engagement with said screw member.

5. The apparatus of claim 4 in which said means for keeping the gear engaged with the screw is a flat, resilient strip, said bottom shell member has interior walls forming a recess generally rounded at one end and communicating with said screw member and having room in said rounded portion of said recess to receive said gear, the walls forming said recess of the side remote from said screw member having end support portions receiving the end of said flat resilient strip and holding the flat surface of said strip in contact with the teeth of said gear and a central open portion between said end support portions to permit said resilient strip to flex when said gear is pushed away from said screw.

6. The apparatus of claim 1, in which the resistor is bonded to a vitreous surface.

7. The apparatus of claim 1, in which the resistor is bonded to a ceramic surface. 

1. A trimming potentiometer comprising a housing including a top and bottom shell number, a screw member rotatably mounted in the bottom shell member, a gear removably positioned within said bottom shell member and engaging said screw member and having on one of its sides a laterally disposed cam, a contact drum containing on one side an electrical contact and on the others a groove in engagement with the gear cam thereby enabling the gear and drum to rotate as a single unit, a resistance element removably fixed within the top shell member and communicating wIth the electrical contact of the drum, the resistance element being further characterized by having an arcuate shaped resistor coated on its surface and a highly conductive element coated at each end thereof forming an angle of less than 360* and a central conductor out of contact with the other resistor and conductive parts coated on the center part of the resistance element, means by which the contact drum cannot rotate more than 360*, means for keeping the gear engaged with the screw and for flexing with the gear when it is forced out of engagement with the screw member, means for reducing friction when the drum rotates against part of the bottom shell and electrical means for connecting the highly conductive parts of the resistance element to a power source.
 2. The apparatus of claim 1 wherein said means for keeping the gear engaged with the screw comprises a spring positioned within the bottom shell in proximity to the gear whereby it will flex whenever the gear is forced out of engagement with the screw member and otherwise serve to keep the gear in engagement with the screw.
 3. The apparatus of claim 2 in which said spring is a flat, resilient strip held in contact with the teeth of said gear.
 4. The apparatus of claim 1 in which said laterally disposed cam is wedge shaped and is so oriented with respect to said gear, said electrical contact, and said resistor that each flat surface of said cam is substantially perpendicular to said screw member when said gear is in either of two end positions to place said contact upon a respective end of said resistor, whereby further rotation of said screw member will force said cam out of said groove in said contact drum to permit said gear to overcome the pressure of said means for keeping the gear engaged with the screw and move out of engagement with said screw member.
 5. The apparatus of claim 4 in which said means for keeping the gear engaged with the screw is a flat, resilient strip, said bottom shell member has interior walls forming a recess generally rounded at one end and communicating with said screw member and having room in said rounded portion of said recess to receive said gear, the walls forming said recess of the side remote from said screw member having end support portions receiving the end of said flat resilient strip and holding the flat surface of said strip in contact with the teeth of said gear and a central open portion between said end support portions to permit said resilient strip to flex when said gear is pushed away from said screw.
 6. The apparatus of claim 1, in which the resistor is bonded to a vitreous surface.
 7. The apparatus of claim 1, in which the resistor is bonded to a ceramic surface. 